In a copending and commonly-assigned U.S. patent application of the same inventor (Shiratsuchi), Ser. No. 38,260, circuitry is disclosed for magnifying a video image in real time. The aforesaid Shiratsuchi patent application is incorporated herein by reference. In an embodiment disclosed therein, the magnification factor MF is varied between 1 and 2 in small increments (i.e., 1/256) to produce the illusion of a continuous zoom.
In the Shiratsuchi zoom system, digital samples representative of a composite video baseband signal CVBS are stored in a digital field memory. The portion of the original image that is to be magnified is first extended in the vertical direction by repeating selected horizontal lines as the signal is read out from the field memory. For example, for a 2-to-1 vertical expansion (i.e., MF=2), each original horizontal line in the magnified portion of the image is read out twice (i.e., L.sub.1, L.sub.1, L.sub.2, L.sub.2, L.sub.3, L.sub.3 and so on).
A Y/C separator coupled to receive the vertically-expanded output of the field memory separates the memory output into a luma signal component Y and a chroma signal component C. For a 2-to-1 vertical magnification, the luma and chroma signals Y and C have the following sequencies: EQU Y=L.sub.2 +L.sub.1, L.sub.2 +L.sub.1, L.sub.3 +L.sub.2, L.sub.3 +L.sub.2 . . . , EQU C=L.sub.2 -L.sub.1, L.sub.2 -L.sub.1, L.sub.3 -L.sub.2, L.sub.3 -L.sub.2 . . . ,
where L.sub.1, L.sub.2, L.sub.3 . . . represent successive horizontal lines of samples of a raster scanned image.
In an NTSC-type system, the color difference signal components (e.g., R-Y and B-Y) are modulated on a color subcarrier signal. The unmodulated frequenty F.sub.sc (3.58 MHz) of the color subcarrier signal is established at an odd multiple (455) of one-half the horizontal line frequency F.sub.H (15,734 Hz) to minimize the visibility of the interference between the recovered luma and chroma signals Y and C. A few cycles (8 to 10) of the unmodulated color subcarrier signal (known as the color burst signal) are included in the composite video baseband signal CVBS during the back porch interval of the horizontal blanking signal to allow regeneration of the color subcarrier signal in the TV receiver. The regenerated color subcarrier signal is used in the TV receiver for demodulating the separated chroma signal C into the respective color difference signals, as is well known in the art.
Because the color subcarrier frequency F.sub.sc is selected to be an odd multiple of one-half horizontal line frequency F.sub.H, the phase of the recovered color subcarrier signal changes by 180.degree. on a line-to-line basis. This feature of the color subcarrier signal facilitates chroma decoding in the TV receiver.
However, when the original image is extended vertically and horizontally by respectively repeating the horizontal lines and pixels, the phase of the modulated color subcarrier signal (i.e., chroma signal C) is disturbed. This makes it difficult to decode the composite video baseband signal CVBS into the luma signal component Y and the two color difference signal components R-Y and B-Y.